MIDIS: JWST/MIRI reveals the Stellar Structure of ALMA-selected Galaxies in the Hubble-UDF at Cosmic Noon

TL;DR

This study uses JWST/MIRI imaging to reveal the detailed stellar structure of ALMA-selected galaxies at cosmic noon, showing smoother, more concentrated structures in the near-infrared than in optical wavelengths, informing galaxy formation models.

Contribution

First deep JWST/MIRI imaging of ALMA-selected galaxies at z=0.5-3.6, revealing their stellar structure at rest-wavelengths >1 micron and revising stellar mass estimates.

Findings

Galaxies are more smooth and centrally concentrated in near-infrared than optical.

Effective radii range from 1 to 5 kpc, with Sersic indices mostly near exponential.

Size ratio Re(F560W)/Re(F160W)~0.7 decreases with stellar mass.

Abstract

We present deep James Webb Space Telescope (JWST)/MIRI F560W observations of a flux-limited, ALMA-selected sample of 28 galaxies at z=0.5-3.6 in the Hubble Ultra Deep Field (HUDF). The data from the MIRI Deep Imaging Survey (MIDIS) reveal the stellar structure of the HUDF galaxies at rest-wavelengths of >1 micron for the first time. We revise the stellar mass estimates using new JWST photometry and find good agreement with pre-JWST analysis; the few discrepancies can be explained by blending issues in the earlier lower-resolution Spitzer data. At z~2.5, the resolved rest-frame near-infrared (1.6 micron) structure of the galaxies is significantly more smooth and centrally concentrated than seen by HST at rest-frame 450 nm (F160W), with effective radii of Re(F560W)=1-5 kpc and S\'ersic indices mostly close to an exponential (disk-like) profile (n~1), up to n~5 (excluding AGN). We find an average size ratio of Re(F560W)/Re(F160W)~0.7 that decreases with stellar mass. The stellar structure of the ALMA-selected galaxies is indistinguishable from a HUDF reference sample of galaxies with comparable MIRI flux density. We supplement our analysis with custom-made, position-dependent, empirical PSF models for the F560W observations. The results imply that a smoother stellar structure is in place in massive gas-rich, star-forming galaxies at Cosmic Noon, despite a more clumpy rest-frame optical appearance, placing additional constraints on galaxy formation simulations. As a next step, matched-resolution, resolved ALMA observations will be crucial to further link the mass- and light-weighted galaxy structures to the dusty interstellar medium.